Generally, bicycle gears are formed in a round or elliptic shape and are made of iron or aluminum plates.
Elliptic gears made of aluminum plates are advantageous because they are lightweight and can be manufactured by punching an aluminum plate. If the aluminum plate, as a starting material, has thickness corresponding to that of the gear teeth members to be provided, there is no need, after punching of the plate, to arrange the thickness of the tooth portions for providing teeth members, thus resulting in a simple manufacturing operation to that extent to thereby provide economical mass production. However, the disadvantage is encountered that the base portion of the gear, exclusive of the teeth members has insufficient strength.
To eliminate this disadvantage, the starting material conventionally uses an aluminum plate having a larger thickness than the teeth members to be provided. The aluminum plate is punched by use of a press machine to form an intermediate element with a number of tooth portions on the outer periphery. The intermediate element is then worked on in a circular cutting operation at both side faces of the tooth portions by use of an NC milling machine or a customary engine lathe, so as to reduce the thickness of the tooth portions in comparison with the substantial thickness of the base portion of the intermediate element, thereby forming teeth members.
The NC milling machine, when used to perform a cutting operation on the tooth portions of the elliptic intermediate element formed by punching of the aluminum plate as described above, can make cuttings each of the same length extending radially inwardly from the tips of all the tooth portions on the sides of major and minor axes of the intermediate element, but this operation takes long hours to perform the cutting and is expensive and not suitable for mass production.
Alternatively, a customary engine lathe can be used for the cutting operation of the tooth portions to form the teeth members. This method is cheaper and can reduce the necessary hours for the cutting in comparison with the work by the NC milling machine. However, in this method using the engine lathe, since the tooth portions of the intermediate element which is rotated are cut by a cutting tool, a radially inward surface of the intermediate element (at which surface the cutting by the cutting tool terminates) is made circular, so that the amount of cutting in the radial direction is larger at the tooth portions at the major axis side of the gear than those at the minor axis side, resulting in a problem of lower strength of the teeth member portions at the major axis side.